Introduction
Vitamin A deficiency is a public health problem and occurs clinically and sub-clinically in 156 countries. Adequate amounts of vitamin A are essential for growth, reproduction, proper functioning of the immune system and vision, among other metabolic functions. Vitamin A deficiency is related to decreased resistance to infections and increased child mortality rates (Brasil, 2013a).
In a study carried out in Brazil with 337 rural children of pre-school age enrolled in municipal schools, Farias et al. (Reference FARIAS, SILVA, SILVEIERA, CALDEIRA and PINHO2015) determined that the amount and frequency of egg consumption was not sufficient to meet the requirements for vitamin A, as those reporting daily egg consumption was only 26.7%.
Considering that the provision of large doses of vitamin A is a short-term solution and that there is a need for adequate dietary intakes from the gestational period onward, it is relevant to consider that a change in eating habits to increase concentrations of vitamin A in the diet should be encouraged. Such a change can be made through informing the population, encouraging the frequent consumption of foods high in vitamin A, mainly of animal origin. In general, these are introduced late in the diet of children and are little consumed throughout adult life due to misinformation relating to e.g. cholesterol content, as is the case for chicken eggs. In addition, there is an increase in the supply of vitamin A through fortified foods (West and Mehra, Reference WEST and MEHRA2010; Sandjaja et al., Reference SANDJAJA, JAHARI, HTET, TILDEN, SOEKARJO, UTOMO and KORENROMP2015).
Makrides et al. (Reference MAKRIDES, HAWKES, NEUMANN and GIBSON2002), in a study carried out with 6 to 12-month-old children consuming four egg yolks per week of common or omega-3 enriched eggs, observed that there was no interference in the consumption of other foods, nor an increase in plasma cholesterol; besides, there was a slight increase in iron for those consuming eggs compared to the group that did not. The authors observed that it is possible and practical to consume the required amount of eggs, even for children at such a young age. This fact goes against what is still disseminated about eggs, where consumption is related to the misinformation about the potential health risks.
The production of enriched eggs could be an affordable way of providing a greater amount of vitamin A to minimise the effects of low or irregular frequency of consumption of table eggs and other sources of vitamin A of animal origin. Thus, vitamin A requirements could be met more easily in the long term, along with the requirements for other essential nutrients related to vitamin A deficiency.
Characterisation of vitamin a and its precursors
The term ‘vitamin A’ refers to a group of retinoid compounds that have properties similar to those of retinol, such as retinal, retinyl esters and retinoic acid, and these are only present in products of animal origin. In plant tissues, some carotenoids, after oxidative degradation in the animal organism, can create retinol (Rutz et al., Reference RUTZ, ANCIUTI and MAIER2014).
Carotenoids are chemically classified into two groups, carotenes and xanthophylls. The most important pro-vitamin A carotenes are β-carotene, β-cryptoxanthin, and α-carotene. Of these, β-carotene is efficiently converted to retinol due to the two β-ionone rings in its structure, which form two retinol molecules. However, α-carotene and β-cryptoxanthin have only one β-ionone ring in their structure, presenting approximately half of the β-carotene pro-vitamin A activity (Shete and Quadro, Reference SHETE and QUADRO2013; García-de Blas et al., Reference GARCÍA-DE BLAS, MATEO and ALONSO-ALVAREZ2015).
The pigment of carotenoids ranges from yellow to red and is used on an industrial scale in food, pharmaceuticals, feed, cosmetics and dietary supplements. Since diets are assumed to provide adequate levels of vitamin A, carotenoids are not considered essential nutrients, but they are used in food enrichment due to their pro-vitamin activity as well as their potential application as antioxidants and immune system enhancers (Green and Fascetti, Reference GREEN and FASCETTI2016).
Sources of vitamin A are abundant in retinyl esters, such as liver, fish, eggs, and certain commercially fortified foods. In plants, carotenes accumulate in the tissue to assist in the absorption of sunlight and to protect it against photooxidative stress. Vitamin A precursors can be found in dark leafy vegetables, carotenogenic tubers such as carrots, and in fruits (Rodriguez-Amaya et al., Reference RODRIGUEZ-AMAYA, KIMURA and AMAYA-FARFAN2008; West and Mehra, Reference WEST and MEHRA2010; Shete and Quadro, Reference SHETE and QUADRO2013; Green and Fascetti, Reference GREEN and FASCETTI2016).
Bioavailability of vitamin A and β-carotene
Bioavailability is defined as the amount of vitamin A that is absorbed, whether in the form of retinyl esters or carotene, and is available for metabolic processes. Its determination involves factors such as the susceptibility to oxidation, processing method, presence of other components in the diet such as fibres and fats, nutritional state of the individual, and parasitic infections and infestations, among others (Rodriguez-Amaya et al., Reference RODRIGUEZ-AMAYA, KIMURA and AMAYA-FARFAN2008). Carotenes are highly reactive due to their conjugated double bond system, which creates the chromophore characteristics; however, this makes carotenes more susceptible to oxidation. This is manifested as a loss of chemical properties by heat, light, oxygen, and enzymes, decreased colour and the inability to form vitamin A. In addition, the denaturation of the cellular wall proteins complexed to the carotene increases its bioavailability, improving solubilisation in the lipid micelles and absorption by enterocytes (Rodriguez-Amaya et al., Reference RODRIGUEZ-AMAYA, KIMURA and AMAYA-FARFAN2008; Van Loo-Bouwman et al., Reference VAN LOO-BOUWMAN, NABER and SCHAAFSMA2014). Dietary fat composition may affect vitamin A bioavailability because its presence in the gastrointestinal tract stimulates pancreatic and biliary secretions, both which are necessary for the formation of micelles and the solubilisation of retinol and carotenoids in the intestinal lumen. The greater the incorporation of the liposoluble vitamins in the micelles, the greater the absorption efficiency (Conaway et al., Reference CONAWAY, HENNING and LERNER2013; Reboul, Reference REBOUL2013).
The bioavailability of preformed vitamin A in the egg ranges from 90 to 100%, accounting for about 15% of the daily consumption recommendation for humans, while for precursor carotenes, bioavailability may vary from 10 to 67%, depending on the source. Thus, vitamin A intake of animal origin is more efficient than that of carotenes (Van Loo-Bouwman et al., Reference VAN LOO-BOUWMAN, NABER and SCHAAFSMA2014).
In Western societies, the largest source of vitamin A, approximately 70% of daily consumption, is from animal origin and preformed vitamin A, either from natural sources or from a concentrated supplement, which is efficiently stored in the body. However, the risk of intoxication increases when preformed vitamin A is provided as a supplement in comparison to natural sources of animal products, such as in the egg (Buss et al., Reference BUSS, TEMBE, PRENDERGAST, RENWICK and GEORGE1994; West and Mehra, Reference WEST and MEHRA2010; Oliveira, Reference OLIVEIRA2015).
Hypovitaminosis and hypervitaminosis A
Hypovitaminosis A is a primary or secondary dietary deficiency of vitamin A and leads to poor resistance to infections, epithelial damage, body growth, raises mortality rate, and partial or total loss of vision (Akhtar et al., Reference AKHTAR, AHMED, RANDHAWA, ATUKORALA, ARLAPPA, ISMAIL and ALI2013). In addition, hypovitaminosis impairs the formation of rhodopsin and causes xerophthalmia, which is the inability to adapt to low levels of light and is an early indication in the diagnosis of vitamin A deficiency in humans (Nelson and Cox, Reference NELSON and COX2014). Symptoms of vitamin A deficiency are similar in birds and mammals, manifested as decreased animal performance and egg production (Cortes et al., Reference CORTES, TIWARY, PUSCHNER, CRESPO, CHIN, BLAND and SHIVAPRASAD2006; İlhan and Bülbül, 2016). In a study that evaluated vitamin A deficiency in turkeys, motor incoordination, inappetence, atrophied growth, atrophied nasal and lacrimal glands, depression and increased mortality were observed. Other symptoms were hyperkeratinisation of the skin, squamous metaplasia of the oral cavity and oesophagus and in the pulmonary bronchi and proventriculus. Squamous metaplasia and anorexia are the main symptoms of vitamin A deficiency in birds (Cortes et al., Reference CORTES, TIWARY, PUSCHNER, CRESPO, CHIN, BLAND and SHIVAPRASAD2006).
When evaluating the effect of vitamin A deficiency on the small intestine of rats, inflammation in enterocytes, decreased villi height and increased crypt depth were observed. Furthermore, mRNA expression for brush-border enzyme synthesis was lower throughout the small intestine, and the authors suggested that this explained alterations in digestion and nutrient absorption in vitamin A deficiency situations (Zaiger et al., Reference ZAIGER, NUR, BARSHACK, BERKOVICH, GOLDBERG and REIFEN2004).
In relation to vitamin A deficiency in birds, various studies have reported ataxia, sore eyes, mortality (Baird et al., Reference BAIRD, RINGROSE and MACMILLAN1939), abnormal bone growth (Havivi and Tal, Reference HAVIVI and TAL1973), increased lactic acid synthesis in the liver (Nockels and Phillips, Reference NOCKELS and PHILLIPS1971), decreased liver weight and increased hepatic glycogen (Sundeen and Richards, 1980), decreased sperm motility and sperm count, and the occurrence of morphological abnormalities that reduced fertility (Paredes and Garcia, Reference PAREDES and GARCIA1959). In a study that evaluated the infection by Eimeria acervulina in chickens with vitamin A-deficient diets, recovery was slower and mortality was higher (Coles et al., Reference COLES, BIELY and MARCH1970). Severe intestinal damage and dehydration have been reported, and birds of all ages ingesting low concentrations of vitamin A showed consistently higher and more prolonged oocyst production following a primary infection with E. acervulina than their counterparts on adequate rations.
Vitamin A is a requirement for embryonic development and is necessary for the prevention of growth abnormalities (Chen et al., Reference CHEN, JIANG, JIANG, LI, LIN, GOU and FAN2015). When evaluating vitamin A supplementation in one-day-old chicks, Fan et al. (Reference FAN, LIU, LIU, ZHAO, JIAO, WANG, SONG and LIN2015) found an increase in the concentration of immunoglobulin A with increased vitamin A intake and therefore supplementation was reported as relevant to the immunity of birds.
Vitamin A inclusion in the diet can minimise the effects of heat stress on laying hens and strengthen the immune system against parasites, viruses and bacteria as it exerts a protective effect on mucous membranes and adrenal glands. In addition, retinoic acid increased the response of T-lymphocytes (El-Hack et al., Reference EL-HACK, MAHROSE, ASKAR, ALAGAWANY, ARIF, SAEED, ABBASI, SOOMRO, SIYAL and CHAUDHRY2016). Khoramabadi et al. (Reference KHORAMABADI, AKBARI, KHAJALI, NOORANI and RAHMATNEJAD2014) reported that vitamin A supplementation in broilers reduced intestinal epithelial erosions and increased weight gain as well as protein and lipid digestibility. Idi et al. (Reference IDI, PERMIN, JENSEN and MURRELL2007) observed that infection by Ascaridiagalli was inversely proportional to the vitamin A level in the diet of laying hens.
The occurrence of hypervitaminosis A is rare in animal production and in humans, and reports in the literature are scarce. According to Guo et al. (Reference GUO, YAN, SHI and FENG2011), excess vitamin A can decrease bone mineral density and growth in broiler chickens. In humans, hypervitaminosis A is usually associated with the excessive intake of retinoids and causes dizziness and convulsions. Chronic intoxication causes cognitive decline, irritability, mental confusion, anxiety, and depression (Conaway et al., Reference CONAWAY, HENNING and LERNER2013; Oliveira, Reference OLIVEIRA2015).
Enrichment of table eggs with vitamin A
Eggs are a source of essential nutrients that nourish the chicken embryo. Considered a functional food, eggs contain compounds such as bioactive peptides, vitamins and minerals, antioxidants, and fatty acids, are nutritionally dense, and the nutrients have a high bioavailability. Eggs can be enriched with vitamins, minerals, and fatty acids by nutritional manipulation of the laying hens’ diet (Sirri and Barroeta, Reference SIRRI and BARROETA2007). Eggs are the lowest-cost source of protein, calcium, zinc, riboflavin, vitamin B12 and vitamin A (Adam, Reference ADAM2010).
An experiment was conducted to evaluate the effects of different dietary vitamin combinations on egg quality and vitamin concentrations in the eggs of commercial laying hens. Results showed hens that received the fortified vitamin had an increased egg deposition of vitamin B12, folic acid, vitamin A, vitamin D, 25-OH-D3, vitamin E, vitamin B1, biotin and pantothenate (Zang et al., Reference ZANG, ZHANG, DING, BAI, HERNÁNDEZ and YAO2011).
In another study, layers were fed diets with regular or three to 10 times more supplemental vitamins, in an attempt to enrich eggs with these nutrients. Dietary concentrations of vitamins were chosen with the expectation of producing an egg with sufficient vitamins to achieve 50% of daily recommended intake for humans. The results showed considerable variation in egg vitamin enrichment, however 50% of the daily recommended dietary intake of biotin, vitamin B12, and vitamin K for humans was achieved with one egg. With a basal diet of 10,000 IU kg vitamin A content enriched with 40,000 IU/kg, there was an increase from 59 to 75 µg in 60 g eggs (Leeson and Caston, Reference LEESON and CASTON2003).
The yolk represents about 31% of the egg. Due to its lipophilic nature, vitamin A is present in the yolk, 99% being in the fat, and absent in the albumen, likewise for vitamin K (0.119 UI/egg), D (18.3 UI/egg), E (0.684 mg/egg) (Bertechini and Mazzuco, Reference BERTECHINI and MAZZUCO2013). The fat content consists of lipids such as triglycerides, phospholipids, and cholesterol besides water, proteins, carbohydrates, and minerals. In addition, high- and low-density lipoproteins are deposited, containing carotenoids. Approximately 40 to 45% of the carotenoids present in the liver of the birds are deposited in the egg yolk (Bortolotti et al., Reference BORTOLOTTI, NEGRO, SURAI and PRIETO2003). Vitamin A is important for metabolic functions in the body; the plant sources are precursors of vitamin A, not preformed, which is found only in products of animal origin, such as the egg.
The vitamin A average content of the egg is 79 µg/100 g or 263 UI/100 g (UNICAMP, 2011) but the values vary. According to Bertechini and Mazzuco (Reference BERTECHINI and MAZZUCO2013) the average concentration of vitamin A in the whole egg is 245 UI/egg and 21 µg/egg of carotenoids. Approximately 50 g of boiled egg provides about 95.5 μg of retinol equivalent (Song and Kerver, Reference SONG and KERVER2000), including both preformed retinoids and precursor carotenoids. Aguiar et al. (Reference AGUIAR, ZAFFARI and HÜBSCHER2009) observed a lack of consensus about vitamin A content in eggs as many factors affect its metabolism and deposition in the yolk. Anderson (Reference ANDERSON2013) observed differences in vitamin A content in eggs from different breeds. Vitamin A content in white eggs was higher at 275.8 UI/50 g and in brown eggs was 178.5 UI/50 g, a difference of 97.3 UI/50 g. The reason for the variation in vitamin A concentration in egg yolk is related to its inclusion in the diet of laying hens, which is also related to the raw materials used in the diet (Sirri and Barroeta, Reference SIRRI and BARROETA2007; Bertechini and Mazzuco, Reference BERTECHINI and MAZZUCO2013).
It is necessary to feed animals adequate levels of vitamins to obtain maximum performance. Few changes have occurred in vitamin requirements for monogastric animals over the years, despite major increases in growth performance, therefore, there is a lack of information about this issue in the literature. According to Mendonça et al. (Reference MENDONÇA, ALMEIDA, MORI and WATANABE2002) the recommended concentration of vitamin A for laying hens was 4,000 IU/kg feed. Currently, the concentration of vitamin A recommended for laying hens is 900 UI/day or 9,000 UI/kg feed and 15,000 UI/kg of egg mass (Rostagno et al., Reference ROSTAGNO, ALBINO, HANNAS, DONZELE, SAKOMURA, PERAZZO, SARAIVA, TEIXEIRA, RODRIGUES, OLIVEIRA, BARRETO and BRITO2017). In another experiment, two different levels of vitamin A supplementation (3,000 and 9,000 IU/kg) were used to investigate laying performance and antibody titre against Newcastle disease virus of heat-stressed hens (Lin et al., Reference LIN, WANG, SONG, XIE and YANG2002). In another experiment by the same researchers, the effect of four levels of vitamin A (3,000, 6,000, 9,000, and 12,000 IU/kg) on the antibody titre to Newcastle disease and T lymphocyte proportion was studied. The results showed that supplementation of commercial diets with vitamin A was beneficial to laying performance and immune function of layer hens (Lin et al., Reference LIN, WANG, SONG, XIE and YANG2002).
Although concentrations of the yolk components are genetically influenced, the efficiency of vitamin A transfer from the diet to the eggs is high and can be achieved by adding preformed vitamin A or its precursors to the diet (Naber, Reference NABER1993; Liu et al., Reference LIU, DAVIS, SCHMAELZLE, ROCHEFORD, COOK and TANUMIHARDJO2012). In a study to evaluate the effect of diets supplemented with retinyl acetate, Mori et al. (Reference MORI, MENDONÇA, ALMEIDA and PITA2003) verified that yolk retinol concentration was enhanced by added vitamin A, which increased from 24.6 IU/g (0 supplementation) to 33.6 and 37.7 IU/g of yolk when hens were fed 15,000 and 30,000 IU/kg of diet respectively. The nutritional value of eggs, related to retinol can be improved by dietary manipulation of the hen's diet, but attention must be focused on the inter-relationship with tocopherol (Mori et al., Reference MORI, MENDONÇA, ALMEIDA and PITA2003), in terms of antioxidation and protection of the vitamin A.
The purpose of enriching a food should be driven by the need to increase the concentrations of beneficial and/or commonly deficient nutrients in the diet of the human population or specific at-risk groups. This process should prioritise the final quality of the product and, preferably, its implementation should be simple and cost-effective. In a study by Cao et al. (2013), students received 200,000 IU vitamin A and one egg per day for six months and were compared to a control group that did not receive any supplementation. Supplementation revealed that consuming one egg and vitamin A supplementation was an effective, convenient, and practical method to improve the levels of haemoglobin, serum retinol and prevent the deterioration of growth in terms of weight gains.
Eggs, as well as meat and other products of animal origin, enriched with vitamins and other substances via feed are known as designer foods - food produced for a specific purpose, which can be achieved through the nutritional management of birds (Kralik and Kralik, Reference KRALIK and KRALIK2017).
Conclusions
Table eggs can play an important role as a functional food because they contain substances that bring physiological benefits, which go beyond providing basic nutritional needs. The inclusion of eggs in the human diet may partially meet the daily intake of protein and vitamins, but the frequency of consumption of vitamin A from foods from animal origin, including eggs, is still low in some regions. Table eggs can be enriched with specific nutrients added to the diet of birds, such as vitamin A.
Enrichment of the egg yolk with vitamin A can ensure nutritional support for adults and children and avoid the occurrence of subclinical deficiency, which is difficult to detect, but recurrent in many countries. Therefore, it is necessary that the composition of poultry diets is based on nutritional management strategies and that the effects of the intake of the enriched product on human nutrition are elucidated, thus allowing its implementation on a large scale.
